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The Sioux Narrows Bridge

If you’re like most people, you probably think of wooden bridges as relics from the past-charming in their antiquity as you come upon them on quiet country roads, yet certainly inferior to their modern concrete and steel counterparts. But then there was the Sioux Narrows Bridge…

At 210 feet, it was thought to be the longest single-span wooden bridge in the world, carrying Highway 71 over a channel of the Lake of the Woods in the town of Sioux Narrows.

Opened on July 1st, 1936 the erection of this bridge proved to be one of the major challenges in the construction of Highway 71 between Fort Frances and Kenora.

The bridge itself was a Howe box truss approximately 210 feet long and 27.5 feet wide, constructed largely of Douglas fir pressure treated with creosote and augmented by steel members. The BC fir was a grade known as dense selected structural hewn from trees that would have predated European settlement in Canada. The core and end members which were the internal components of the bridge, ranged in length from the minimum of sixty feet to a maximum of approximately 105 feet.

Construction on the bridge began in 1935 with the bridge being constructed entirely on site. Each of the thousands of pieces of the bridge came with a lead tag indicating what it was and where it was to go. Each piece was unique both with respect to its length and the angle at which it was cut. The entire structure was shipped from BC like a giant jigsaw puzzle. Engineers who examined the bridge contended that the structure required no ‘field cuts’. Evidence would further suggest that the entire structure fit together perfectly and that it was constructed by carpenters and not engineers.

It was a very sophisticated and complex design. The deck system was designed so that it could be cambered; meaning that if in time it should sink in the middle, it could be raised up again by simply putting more tension on a series of metal rods underneath the decks. Also, because wood shrinks, the whole system was totally adjustable and could easily be tightened up.

The bridge was initially designed to last 40 years. It was thought at one time that the bridge would have to be replaced as were its wooden counterparts at Berry Creek and Reed Narrows. This view towards a replacement prompted intense studies to be undertaken to determine the soundness of the bridge. The results of these engineering studies proved to be quite surprising. From the results of testing sixty core samples of wood from the bridge there were no major signs of decay. In addition, tests undertaken to calculate weight and stress which the bridge was able to accommodate were performed up to a loading of approximately 244 000 pounds. This is six times the original design capacity of twenty tones and would have been quite sufficient to handle Ministry of Transportation loads at that time which did not exceed 150 000 pounds.

With the strength of the bridge no longer in question, many improvements were undertaken to ensure its longevity and safety. In 1981 a new deck was installed which incorporated the use of 2” x 6” decking material placed on edge and forced together by hydraulic jacks. This effectively produced a watertight slab. Following this, a continuous asphalt surface was laid over the bridge for the first time. This surface replaced the ‘weak link’ in the bridge which was the asphalt planking formerly used.

In addition to this new surface for vehicles, a cantilevered walkway was also constructed on the west side to provide a safe pedestrian crossing. Part of the reason for the bridge’s longevity was the excellent care it received. Every spring the bridge was carefully washed down and flushed of the accumulated winter debris. But one aspect of the winter that was positively beneficial was the use of salt on the roads, which acted to preserve the wood of the structure.

In 2003, a condition survey was conducted on the Sioux Narrows bridge. The study confirmed that the bridge was deteriorating in a numbers of ways, namely: deformations (lateral twisting and moving), decay of timber (resulting in reductions in load carrying capacity), timber fibre crushing, corrosion and component failure (crushing and buckling). It was found that 78% of the bridge members had failed and needed to be replaced. A temporary bridge was installed while construction on the brand new bridge commenced.

In November 2007, the new bridge was finally completed. Timbers from the old bridge were incorporated into the new structure and the truss appearance preserved. A ribbon cutting to dedicate the new bridge was held July 1st, 2008 – 72 years after the original celebration.